The invention concerns a coating composition for use in forming a catalytic porcelain enamel coating on food related devices such as, for example, ovens and microwave cooking devices. More particularly, the present invention concerns a new porcelain enamel coating composition for use on aluminized steel that displays excellent self-cleanability.
In certain food related applications, such as the walls of food cooking ovens, it is advantageous to have a coating on the inside of the oven cavity that affords easy cleaning. Unfortunately, however, most porcelain enamel coating materials exhibit characteristics which are not ideal from the standpoint of cleanability. The prior art does provide porcelain enamel compositions that afford improved cleanability. Examples of such compositions may be found in Saad et al. U.S. Pat. No. 5,382,552 and Becker et al. U.S. Pat. No. 4,204,021. However, such compositions are intended primarily for use upon a plain steel substrate.
In many food related applications, aluminized steel is employed. Nishino et al. U.S. Pat. No. 4,460,630 discloses a porcelain enamel coating system for use on aluminized steel that exhibits improved cleanability. However, the system of the ""630 patent is a two-coat system.
The present invention provides a new and improved porcelain enamel coating composition for use on aluminized steel that may be applied dry as a single coat and it also affords a high degree of cleanability. The coating composition of the present invention is especially well-suited for use in connection with cookware applications. No degreasing of the aluminized steel is required prior to the application of the enamel composition.
In a preferred embodiment the porcelain enamel coating material of the present invention comprises a glass component and one or more mill additions. The glass component includes a composition by weight from about 30% to about 55% SiO2, from about 11% to about 20% TiO2, from about 0% to about 10% B2O3, from about 20% to about 40% alkali metal oxides, less than about 5% rare earth metal oxides, from about 0% to about 10% CaO, from about 0% to about 10% BaO, from about 0% to about 10% MnO, from about 2% to about 12% CuO, from about 0% to about 10% Sb2O3, from about 1.5% to about 10% Fe2O3, from about 0% to about 10% SnO2, from about 0% to about 10% P2O5, less than about 2% MoO3, and less than about 5% V2O5. The glass component may comprise one or more glass frits so as to provide this overall stated composition.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
The novel porcelain enamel composition of the present invention is a significant improvement over prior art compositions because it can be applied to aluminized steel by dry electrostatic spraying and it exhibits excellent cleanability well beyond six (6) cycles (i.e., more than six cycles of heating, cooking and cleaning of burnt-on food from the surface consistent with ISO 8291).
The porcelain enamel composition comprises from about 80% to about 99.9% by weight of the glass component and from about 0.1% to about 20% mill additions. The glass component provides a composition as follows:
The alkali metal oxides include one or more oxides of lithium, sodium, potassium, rubidium and cesium. The rare earth oxides include one or more oxides of the lanthanoids and yttrium and scandium. Generally, the rare earth oxides comprise CeO.
The enamel compositions according to the present invention are suitable for application to a substrate using conventional dry application techniques such as, for example, a standard corona charging gun. Dry application techniques are well-known and discussed in U.S. Pat. Nos. 4,110,487 and 3,928,668. The disclosures of the ""487 and ""668 patents are incorporated herein by reference for their teachings relative to the dry or electrostatic application of porcelain enamel coating compositions.
The coating composition of the present invention may be applied directly to the metal substrate without the need of a ground coat or bonding coat. In general, the coating composition may be applied at a rate of 25 to 80 grams per square foot so as to provide a coating thickness of from about 3 to about 10 mils subsequent to firing.
Articles that have been coated with the coating composition of the present invention may be fired in a conventional manner using conventional firing equipment. Firing is generally conducted at a temperature of 500xc2x0 C. to about 650xc2x0 C. for a period of about 4 to about 13 minutes. Of course, the specific desired temperature for firing will be a function of the grade of aluminized steel employed and the exact composition of the enamel.
The glass frit or frits that comprise the glass component may be prepared utilizing conventional glass melting techniques. A conventional ceramic refractory, fused silica or platinum crucible may be used to prepare the glass frit, molten glass being formed in the crucible and then converted to glass frit using water-cooled rollers and milling equipment. It will be appreciated that the step of producing the glass frits is not per se critical and any of the various techniques well-known to those skilled in the art can be employed.
It will be appreciated that the glass component is used in combination with various mill additions. It will also be appreciated that the mill additions will vary depending upon the specific application conditions being utilized. For dry applications (electrostatic), the glass component is used in combination with one or more conventional organopolysiloxanes. Such organopolysiloxanes are commonly employed in enamel compositions that are designed for dry application. Although the coating composition is intended primarily for dry application, it will be appreciated that for electrophoretic applications and other wet application processes such as spraying and dipping, the glass frits used may be milled in conjunction with other mill additions such as quartz, bentonite, etc. to produce a suitable composition. As is well-known in the enamel art, there is a wide range of other acceptable mill agents or components that may also be utilized in the present invention to produce the desired resultant product.
The resultant product preferably provides a coarseness of 4% by weight through a 325 mesh sieve (45 xcexcmicrons). Fortunately, the milling fineness can be altered without a significant impact on the final coating.
The following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims.